api: handle newlines in JobSubmission vars correctly (#23560)

Fixes a bug where variable values in job submissions that contained newlines
weren't encoded correctly, and thus jobs that contained them couldn't be
resumed once stopped via the UI.

Internal ref: https://hashicorp.atlassian.net/browse/NET-9966

command/agent/job_endpoint.go

@@ -908,6 +908,8 @@ func apiJobSubmissionToStructs(submission *api.JobSubmission) *structs.JobSubmis
 	if submission == nil {
 		return nil
 	}
+
+	submission.Canonicalize()
 	return &structs.JobSubmission{
 		Source:        submission.Source,
 		Format:        submission.Format,